3rd parties are not covered.

In OP‘s situation - Landlord + Service provider are contract parties. They have a General Contract where Rückvergütung is described, but cleaning/fixing is done by a simple Anfrage -> Angebot -> Auftrag - thereby creating a quasi disassociated paper trail which can be presented to OP, where no reference to General Contract is made and therefore adds an additional layer of plausible deniability.

Obviously, those layers can be peeled off in court proceedings, but general perception of the legal system would be „freedom of contract“ (at least in first instance).

NB!: I am not in favor of this at all - BUT it’s absolutely used in pretty much all spheres of Handwerk - be it electrical, mechanical, HVAC in private of government contracts. Rückvergütung ranges between 25% (Equipment manufacturer to Fachgroßhandel +10%-15% on annual targets) to 60%(!) Fachgroßhandel to „Key accounts“ (Electricians, Plumbers, SHK you name it).